This invention relates to weigh scales. More particularly, it involves techniques for protecting load sensing elements in weigh scales.
In commonly assigned U.S. Pat. No. 3,938,603, there is disclosed a weigh scale utilizing a floating flexure beam which senses the weight of a load placed on the scale. Briefly, the patented scale includes a load receiving pan or platform spaced from the base and a pair of pivot members. The beam is connected between the pivot members. Turning moments are generated in the pivot members when a load is applied to the scale platform. The turning moments cause the beam to deflect as a function of the load. Sensing means such as strain gauges are applied to central portions of the beam to provide an indication of the amount of beam deflection.
The proportional deflection of the beam with respect to the load is one of the most critical aspects of the operation of the scale. If the beam is allowed to deflect to such a degree that it exceed its elastic limits, the beam will stretch or distort such that inaccuracies would result in subsequent measurements. This phenomena is known as yielding. Although the scales are rated according to their maximum capacity, they are often subjected to excessive loads which could cause yielding and thus destroy the precise accuracy of the scale.
Preloaded springs on the rails supporting the platform have been used to prevent further rotation of the pivot members when excessive loads are applied. The preloaded springs are located about the four corners of the scale and serve to bottom out on the base after their preload strengths have been exceeded. While satisfactory protection has been provided by this technique, it does not readily lend itself to implementation in large capacity scales. This technique also may not respond quickly enough to protect against shock loads applied to the scale platform. Furthermore, a compromise in the preload strengths of the springs must be reached to provide sufficient protection against excessive off-centered as well as centered loads on the scale.